This invention pertains generally to metallic materials, and, more particularly, to a method for inhibiting grain growth and restricting grain size during heat-treatment and hot-working of metallic materials. As used herein, the term "metallic material" includes pure metals, metal alloys, and intermetallic alloys among others.
Annealing and deformation processing of metallic materials at high temperature can provide unique phase morphologies by solutioning primary phases and controlling their precipitation upon cooling. In addition, increasing the temperature at which a specific metallic material is processed can reduce forming power requirements since material formability is increased. However, exposure of metallic materials to high temperatures results, in many cases, in deleterious grain growth. Although a coarse grain structure is desirable from a fracture behavior and high temperature strength standpoint, a large grain size in the final product can negate desirable phase morphologies and dominate mechanical behavior by causing poor fatigue resistance and low room temperature strength and ductility. Using lower temperatures to prevent deleterious grain growth results in higher power requirements for processing equipment due to the reduced formability of metals. At these lower temperatures, processing time is extended because lighter passes must be taken, and more intermediate workpiece conditioning is required because the occurrence of surface-cracking is increased.
The current method for inhibiting grain growth during heat-treatment and hot-working of metallic materials is to establish low solubility dispersoids or precipitates in the metallic matrix. The dispersoids have good thermal stability so they resist dissolution in the matrix during extended high temperature exposure. Dispersoids can be introduced into the matrix by traditional alloying during ingot melting operations, or by solid state powder metallurgy processes such as mechanical alloying. At elevated temperatures, these incoherent dispersoids interact with grain boundaries to prevent grain boundary movement and corresponding grain growth. In many alloy systems, when thermally stable dispersoids are not possible for grain boundary control, matrix precipitates are employed to inhibit grain boundary movement during thermal exposure. In either case, however, processing and heat-treatment ,are limited to temperatures where sufficient thermal stability of the dispersoids or precipitates exist. As temperatures approach the solvus of the dispersoids or precipitates, and the matrix diffusivity of their constituent elements increase, the dispersoids or precipitates will coarsen, coalesce, and/or dissolve into the matrix. For example, alpha-beta titanium alloy Ti-6 wt % Al-4 wt % V (Ti-6-4) component workpieces are processed and heat-treated at least 50.degree. F. below the alloy's beta transus (1820.degree. F.) to allow primary alpha precipitates to remain present in the matrix at peak temperatures to inhibit the growth of beta grains. Heating above the alloy's beta transus will result in solutioning of primary alpha particles, allowing unrestricted growth of beta grains. Accordingly, line grain size is currently preserved by heating and processing just below the solvus of a secondary phase.
There is therefore a need in the art for a method of maintaining a fine grain size during heat-treatment and processing of metallic materials at extremely high temperatures.